1. Field of the Invention
The present invention relates to an image forming apparatus and a control method of an image forming apparatus and, more particularly, to an image forming apparatus for forming an image by electrophotography and a control method of the image forming apparatus.
2. Description of the Related Art
Conventionally, for a so-called electrophotographic image forming apparatus which forms an electrostatic latent image on an image carrier by laser exposure, develops the latent image using developing agents including toners of predetermined colors, and transfers the image to a printing medium to form a visible image, a pulse width modulation (PWM) technology is known, in which gradation can be accurately expressed by modulating the pulse width of a laser beam in accordance with an image signal.
In the image forming apparatus using PWM, a patch based on a predetermined exposure amount is formed on the image carrier. On the basis of the measurement value of the patch density, the mixing ratio (T/C ratio) of two components (toners and carriers) the developing agents or the contrast potential for developing is controlled, thereby maintaining given gradation characteristics of the apparatus.
On the other hand, the maximum density in image formation can be controlled by changing the maximum pulse width in PWM.
However, if a maximum pulse width Pff of the laser beam is changed to change the maximum density of an image and the exposure amount in forming a patch changes accordingly, accurate patch control (calibration using a patch) would be difficult, and given gradation characteristics cannot be maintained.
For example, when the maximum pulse width Pff is increased to increase the maximum density, the exposure amount of a patch portion also increases to result in a high density measurement value of the formed patch. In gradation control based on patch density, for example, the T/C ratio is reduced to maintain a predetermined patch density. More specifically, although the maximum pulse width Pff is increased to increase the entire image formation density, the T/C ratio is reduced, i.e., inverse correction is done. This may pose a fatal problem such as carrier adhesion in the printing medium. In contrast potential control based on patch density as well, inverse correction is performed: although the density need be made high, a contrast potential Vcont is reduced.
When the maximum pulse width Pff is decreased to lower the maximum density, inverse correction is performed such that the T/C ratio is increased. This poses a problem of toner scattering or fog.
The present invention has been made to solve the above-described problem, and has its object to provide an image forming apparatus capable of maintaining stable gradation characteristics even when the maximum pulse width is changed in image formation by pulse width modulation of a laser beam, and a control method of the image forming apparatus.
In order to achieve the above object, the image forming apparatus according to the present invention has the following arrangement.
According to the present invention, there is provided an image forming apparatus characterized by comprising modulation means for pulse-width-modulating an image signal to output a pulse width modulation signal, setting means for setting a maximum pulse width of the image signal in the modulation means, formation means for exposing and developing an image on an image carrier on the basis of the pulse width modulation signal to form a visible image, generation means for generating a predetermined pattern using the formation means, correction means for correcting an image formation condition on the basis of the density of the generated predetermined pattern, and control means for controlling, in forming the predetermined pattern, to obtain a predetermined exposure amount on the image carrier independently of the maximum pulse width set by the setting means.
Other features and advantages of the present invention will be apparent from the following description taken in conjunction with the accompanying drawings, in which like reference characters designate the same or similar parts throughout the figures thereof.